A cemented carbide alloy, which contains tungsten carbide as a main component and cobalt, nickel, or the like as a binder metal and which has added thereto a carbide of titanium, tantalum, chromium, or the like for improving performance, has been widely used in tools for metal processing or the like by virtue of its excellent hardness and abrasion resistance.
In the tools using such cemented carbide alloy, a tool which cannot be used any more due to a defect, abrasion, or the like during use or a defective part thereof is discarded as scrap called hard scrap.
Further, part of cemented carbide alloy powder generated during manufacturing of a cemented carbide tool, ground dust generated during processing of the cemented carbide tool with a grinding stone, and the like are discarded as scrap called soft scrap.
It should be noted that, in the following description, the “cemented carbide scrap” refers to used scrap of an alloy containing 50 wt % or more of tungsten carbide, and cobalt or nickel as a binder phase.
The hard scrap and soft scrap each contain a large amount of tungsten, which is a rare metal. 60% or more of tungsten resources have been used in the cemented carbide tools. Further, the prices of ammonium paratungstate (APT) and tungsten oxide serving as intermediate raw materials for a tungsten material have continued to rise in recent years, and there is a demand for establishment of a recycling technology for tungsten contained in the cemented carbide tools.
For example, in Non Patent Document 1, there is disclosed a method of recycling a cemented carbide tool, which involves recycling tungsten carbide from a used cemented carbide tool or the like. Specifically, the method of recycling a cemented carbide tool of Non Patent Document 1 is performed as described below. First, hard scrap or soft scrap of a cemented carbide tool is allowed to react with a molten salt of sodium nitrate (molten salt dissolution reaction), and the resultant is dissolved in water to produce an aqueous solution of sodium tungstate. Next, an aqueous solution of ammonium tungstate is produced from the aqueous solution of sodium tungstate by an ion exchange method using an ion exchange resin, and ammonium paratungstate (APT) is crystallized out from the aqueous solution of ammonium tungstate. After that, ammonium paratungstate thus crystallized out is subjected to calcination, reduction, and carbonization, with the result that tungsten carbide can be obtained.
On the other hand, in Patent Document 1, there is disclosed treatment of reducing hexavalent chromium to be generated during a reaction between a molten salt of sodium nitrate and cemented carbide scrap to water-insoluble trivalent chromium having low harmfulness. There is also disclosed a method of performing the treatment semi-continuously while controlling the reaction efficiently (Patent Document 1).
Further, in Patent Document 2, there is proposed the use of a molten salt containing 60 wt % to 90 wt % of sodium hydroxide and 10 wt % to 40 wt % of sodium sulfate in production of sodium tungstate by oxidation of scrap of a hard alloy and/or scrap of a heavy metal in a molten salt bath (Patent Document 2). There is also proposed that a reaction between the scrap and the molten salt is performed in a rotary kiln, which is operated batch-wise and can be heated directly.